In laser machining or material processing operations in which pulses of laser radiation are repetitively delivered to a workpiece it is often necessary to change the repetition frequency (PRF) of the pulses while maintaining the peak power of the pulses, or the energy per pulse, constant. In a pulsed laser or a pulsed, master oscillator power amplifier (MOPA) laser-system including a continuously pumped gain-medium a change in repetition rate can result in a change in pulse energy. This is because the interval between the delivery of pulses in a train, among other factors, determines the gain available for each pulse. Further, in a pulsed-laser that is activated to provide a pulse train after a period of inactivity in which the gain medium continues to be pumped, the power of the first pulse in a train can be much higher than any subsequent pulses.
Several techniques for providing variation of the PRF of a pulsed laser, or a pulsed laser and amplifier system while maintaining constant energy per pulse have been described in the prior-art. These techniques include operating the laser or amplifier in a continuous (CW) mode while pulses are not being delivered, allowing the laser or amplifier to operate in a CW mode between pulses in a train, and operating the laser or amplifier at a PRF much greater than the nominal PRF before a pulse train is delivered or between pulses in a train. Examples of such techniques are described in one or more of U.S. Pat. Nos. 6,038,241; 6,414,980; 6,418,154; 6,683,893; and 6,931,035, all of which are assigned to the assignee of the present invention.
The method of carrying of the techniques primarily involve certain sequences of operation of a Q-switch which is normally provided in the laser for causing the laser to operate in a pulsed mode and controlling the PRF of the laser, whether or not any measures were taken for controlling pulse energy or first pulse suppression. Such additional operations include steps in which the Q-switch must be selectively partially opened (or closed). An acousto-optic Q-switch is preferred for the operation of the lasers described. This type of Q-switch is operable in a partially open or closed mode.
Adding such Q-switch operations for pulse-energy control adds to the complexity of circuitry required to control the Q-switch as careful synchronization of the additional operations with “normal” operations is required. Certain amplifier types such as regenerative amplifiers used in ultrafast laser-amplifier systems, require a Pockels-cell type Q-switch, which operates by switching electrically induced birefringence, and does not lend itself to operation in a partially open or partially closed mode. It would be advantageous to have a method for controlling pulse energy and providing first pulse suppression that did not require complex, synchronized Q-switch operations. Preferably such a method would be primarily passive and be effective independent of the type of Q-switch in the laser.